The present inventive concept relates to correlated double sampling (CDS), and more particularly, to a CDS circuit for performing CDS without utilizing a subtractor or an accumulator which performs a subtraction operation, and an image sensor including the same.
CDS is a technique of sampling or reading a signal value with respect to a reference value. CDS may be performed to remove an offset from a signal to attain a pure signal. CDS is widely used in image sensors.
An offset between pixels or channels in image sensors may cause fixed pattern noise to occur in images. CDS may be used to correct this phenomenon. For example, an offset and fixed pattern noise may occur between columns in complementary metal-oxide semiconductor (CMOS) image sensors, which include an analog-to-digital converter (ADC) for each column. CDS may be used to remove the offset and to reduce the fixed pattern noise between the columns.
When CDS is implemented digitally, a subtraction operation is performed to generate a difference between the result of analog-to-digital conversion on a reference value, and the result of analog-to-digital conversion on a signal value. A separate subtractor or an accumulator which performs subtraction may be used to perform this subtraction operation.
When a separate subtractor is used to perform the subtraction operation, the CDS operation includes overhead corresponding to the subtractor and memory. When the accumulator is used to perform the subtraction operation, a circuit configured to perform the subtraction operation is included in the accumulator. When a separate subtractor is provided for each column, or when a subtraction circuit is additionally provided in an accumulator, overhead in image sensors is increased.